As is known, the total base number (TBN) of a lubricating oil is an important indicator of oil condition. For example, during combustion of fuel in an engine, sulfur in the fuel is ultimately converted to sulfuric acid which if not neutralized will significantly increase the wear and corrosion of the engine components.
Lubricating oils are formulated to be highly alkaline in order to neutralize the acids formed during the combustion process. Indeed, lubricating oils are routinely “overbased” with calcium carbonate.
The recommended range of an oil's TBN depends in part on the range of sulfur levels in the fuel being burned. Thus overbased lubricants are particularly important for marine and stationary power plant engine applications because high sulfur fuel is often burned in these engines.
The ability to monitor the TBN of an oil is quite useful in evaluating whether the oil can continue to meet its intended function.
TBN is reported in terms of milligrams of potassium hydroxide (equivalent) per gram of lubricant (mgKOH/g), reflecting the acid neutralizing capacity relative to the strong base. Standard laboratory methods for determing TBN are ASTM D2896 and D4739. The conventional method used in the marine industry for both new and used oils is D2896.
In WO 03/073075 A2 there is disclosed a method for determining the TBN of an oil by measuring one or more selected infrared absorption bands of the oil and correlating the measured bands with the TBN. While this technique is useful in determining an oil's TBN without knowing the TBN of a fresh oil, its accuracy has a lower limit of 20 mgKOH/g.
It would be advantageous to provide a TBN measuring method that is amenable to onsite, online and inline measurement of lubricating oil and that is accurate over the entire range of TBN levels for overbased lubricants, nominally 0 to 100 mgKOH/g.